Astronomers using the MeerKAT telescope at the South African Radio Astronomy Observatory have spotted a pair of enormous radio-emitting bubbles in the central region of our Milky Way Galaxy. The bipolar bubble structure is about 457 light-years wide by 1,403 light-years tall and is likely the result of a phenomenally energetic burst that erupted near Sagittarius A* — the Milky Way’s supermassive black hole — a few million years ago.
Radio image of the central portions of the Milky Way Galaxy. The Galactic plane is marked by a series of bright features, exploded stars and regions where new stars are being born, and runs horizontally through the image. The black hole at the center of the Milky Way is hidden in the brightest of these extended regions. The radio bubbles discovered by MeerKAT extend vertically above and below the plane of the Galaxy. Many magnetized filaments can be seen running parallel to the bubbles. Image credit: University of Oxford / SARAO.
“The Milky Way’s center is relatively calm when compared to other galaxies with very active central black holes,” said co-lead author Dr. Ian Heywood, an astronomer at the University of Oxford.
“Even so, the Milky Way’s central black hole can — from time to time — become uncharacteristically active, flaring up as it periodically devours massive clumps of dust and gas.”
“It’s possible that one such feeding frenzy triggered powerful outbursts that inflated this previously unseen feature.”
Using the MeerKAT telescope, Dr. Heywood and colleagues mapped out broad regions in the center of the Galaxy, conducting observations at wavelengths near 23 centimeters.
Radio emission of this kind is generated in a process known as synchrotron radiation, in which electrons moving at close to the speed of light interact with powerful magnetic fields.
This produces a characteristic radio signal that can be used to trace energetic regions in space.
This radio light easily penetrates the dense clouds of dust that block visible light from the center of our Galaxy.
“The shape and symmetry of what we have observed strongly suggests that a staggeringly powerful event happened a few million years ago very near Sagittarius A*,” said co-author Dr. William Cotton, an astronomer with the National Radio Astronomy Observatory.
“This eruption was possibly triggered by vast amounts of interstellar gas falling in on the black hole, or a massive burst of star formation which sent shockwaves careening through the Galactic center.”
“In effect, this inflated bubbles in the hot, ionized gas near the Galactic center, energizing it and generating radio waves that we could eventually detect here on Earth.”
The environment surrounding Sagittarius A* is vastly different than the environment elsewhere in the Milky Way, and is a region of many mysteries.
Among those are large-scale, highly organized magnetic filaments found nowhere else, the origin of which has remained an unsolved puzzle since their discovery 35 years ago.
The filaments appear as radio structures tens of light-years long and approximately a light-year wide.
“The radio bubbles discovered by MeerKAT now shed light on the origin of the filaments. Almost all of the more than one hundred filaments are confined by the radio bubbles,” said co-author Dr. Farhad Yusef-Zadeh, a researcher at Northwestern University who discovered the filaments in the early 1980s.
The close association of the filaments with the bubbles implies that the energetic event that created the radio bubbles is also responsible for accelerating the electrons required to produce the radio emission from the magnetized filaments.
“These enormous bubbles have until now been hidden by the glare of extremely bright radio emission from the center of the Galaxy,” said co-lead author Dr. Fernando Camilo, an astronomer at SARAO.
“With this unexpected discovery we’re witnessing in the Milky Way a novel manifestation of galaxy-scale outflows of matter and energy, ultimately governed by Sagittarius A*.”
The findings were published in the journal Nature.
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I. Heywood et al. 2019. Inflation of 430-parsec bipolar radio bubbles in the Galactic Centre by an energetic event. Nature 573: 235-237; doi: 10.1038/s41586-019-1532-5
